This hydrometer is the one we use.  
It is super simple to use, makes checking your specific gravities easy!
 
Handy compact size for use with hard-to-get-at batteries
Large, easy-to-read scale with quick-check colored sectors
Readings independant of vertical positioning of instrument
Fast, bubble-free intake of fluid sample
Automatic air-lock prevents fluid loss and air bleed
Unexcelled temperature compensation and measuring accuracy
Sturdy, shock-resistant materials, non-corrosive throughout
 
Dimensions: 5.25" x 4.25" x 1.5"
 
Manual
InstructionalVideo